Room Temperature Low Contact Pressure Method

ABSTRACT

In one or more aspects of the present disclosure, a method is disclosed for fabricating a rigid flex cable assembly. The method includes arranging one or more layers of a polyimide film having an electrically-conductive surface into a stack; applying an adhesive to the one or more layers of polyimide film; arranging a top and a bottom polyimide film to a top portion and a bottom portion of the stack to form the rigid flex circuit board; and selecting a suitable pressure and temperature at which to form the rigid flex assembly such that the one or more layers of polyimide film do not move with respect to one another during the fabrication.

This is a divisional application of U.S. application Ser. No.12/962,206, filed Dec. 7, 2010, entitled “Room Temperature Low ContactPressure Method,” which is incorporated by reference herein in itsentirety.

BACKGROUND

This disclosure relates generally to the field of rigid flex cableassemblies an(more specifically, to a method and a product madeaccording to the method for a room temperature low pressure contactpressure method for rigid flex cable adhesive bonding systems.

Flexible printed circuit boards are widely used in consumer andindustrial appliances and in appliances for telecommunications, Suchboards comprise a flexible, dielectric substrate, one or more conductorscarried on at least one surface of the substrate, a coverlayelectrically insulating the conductors, and one or more electricalcontacts in electrical communication with the conductors and extendingthrough and beyond the coverlay.

A majority of rigid flex cable assemblies are fabricated using alamination process with moderate to high pressure (e.g., 50-250 psi) andrelatively high temperature (e.g., 180-300° C.). When this process isemployed, the cycle time for a material set-up and equipment preparationcan yield a single “qualified” good or acceptable part in an 8 hourperiod. This methodology was developed in the early 1970's and has sincecontinued. The standard high temperature method was developed toeliminate one process step and combine three separate assemblies (rightand left rigid sections and a connecting flexible middle section).However, during fabrication, if any anomalies occur in the process(movement of layers due to pressure and lay-up geometry/tooling, ormovement of layers due to plastic deformation during the cure, orintroduction of delamination due to foreign contaminants), then the costsavings for minimizing the three sections into one assembly is lost. Theundesirable alternative is a high cost remake of the assembly or a timeconsuming manual process requiring specialized repair fixtures.

Given the above problems with the conventional lamination process, whatis needed is a system and method for forming rigid flex cable assembliesusing low pressure bonding process at room temperatures that are lesscostly and cumbersome to produce.

SUMMARY

As described in the various aspects of the present disclosure, a methodof fabricating a rigid flex cable assembly is disclosed. The method caninclude arranging one or more layers of a polyimide film having anelectrically-conductive surface into a stack; applying an adhesive tothe one or more layers of polyimide film; arranging a top and a bottompolyimide film to a top portion and a bottom portion of the stack toform the rigid flex circuit board; and selecting a suitable pressure andtemperature at which to form the rigid flex assembly such that the oneor more layers of polyimide film do not move with respect to one anotherduring the fabrication.

In some aspects, pressure between about 1 psi and 10 psi at atemperature between about 20° and 25° C. can be applied to the rigidflex assembly. The electrically-conductive surface can include copper,gold, nickel, silver, aluminum, composites of copper and tin or leadalloy, composites of copper, nickel and antimony, composites of copper,nickel and gold, and composites of nickel, gold or silver. Theelectrically-conductive surface can be arranged on a top side, a bottomside or both of the polyimide film. The pressure can be between about 2psi and 5 psi and be applied for about 30 minutes. The adhesive caninclude an acrylic-based adhesive, a room temperature curing siliconeadhesive, a solvent blocked silicone adhesive, a flexible anaerobicadhesive, or a frozen preform adhesive. The method can include arranginga first layer of thermoplastic film to the top of the rigid flex circuitboard and a second layer of thermoplastic film to the bottom of therigid circuit board. In some aspects, the thermoplastic film can includea fluoropolymer resin. In some aspects, the method can include preparingthe electrically-conductive surface according to a predetermined circuitconfiguration. A product can then be formed by this method includingrigid flex circuits, flexible cable interconnects, energy absorptivebase plates, packaging/container for shipping applications and variousautomotive products.

In accordance with some aspects of the present disclosure, a method offabricating a rigid flex cable assembly. The method can include applyinga suitable pressure at a suitable temperature to a stack of one or morelayers of a polyimide film having an electrically-conductive surface andan adhesive between the one or more layers of polyimide film to producea rigid flex cable assembly such that the one or more layers ofpolyimide film do not move with respect to one another during thefabrication.

In some aspects, the pressure can be between 1 psi and 10 psi at atemperature of about 23° C. The electrically-conductive surface caninclude copper, gold, nickel, silver, aluminum, composites of copper andtin or lead alloy, composites of copper, nickel and antimony, compositesof copper, nickel and gold, and composites of nickel, gold or silver.The electrically-conductive surface can be arranged on a top side, abottom side or both of the polyimide film. The pressure can be appliedfor about 30 minutes. The adhesive can include an acrylic-basedadhesive, a room temperature curing silicone adhesive, a solvent blockedsilicone adhesive, a flexible anaerobic adhesive or a frozen preformadhesive. In some aspects, a product can be formed by this methodincluding rigid flex circuits, flexible cable interconnects, energyabsorptive base plates, packaging/container for shipping applicationsand various automotive products.

These and other features and characteristics, as well as the methods ofoperation and functions of the related elements of structure and thecombination of parts and economies of manufacture, will become moreapparent upon consideration of the following description and theappended claims with reference to the accompanying drawings, all ofwhich form a part of this specification, wherein like reference numeralsdesignate corresponding parts in the various Figures. It is to beexpressly understood, however, that the drawings are for the purpose ofillustration and description only and are not intended as a definitionof the limits of claims. As used in the specification and in the claims,the singular form of “a” and “the” include plural referents unless thecontext clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded view of an example flex portion of a rigid flexstructure in accordance with aspects of the present disclosure.

FIG. 2 shows the view of FIG. 1 with the addition of an adhesive appliedto the flex portion of the rigid flex structure.

FIG. 3 shows an example low pressure and room temperature bondingapparatus for use with the flex portion of the rigid flex structure ofFIG. 2.

FIG. 4 shows a result of the bonding process of FIG. 3 and thedrilled/metalized via holes.

FIG. 5 shows an example exploded view of a portion of a rigid flexstructure including both the rigid and the flexible sections inaccordance with aspects of the present disclosure.

FIG. 6 shows the view of FIG. 5 with the addition of an adhesive appliedto portions of structure.

FIG. 7 shows an example low pressure and room temperature bondingapparatus for use with the structure of FIG. 6.

FIG. 8 shows a result of the bonding process of FIG. 7 and thedrilled/metalized via holes.

FIG. 9 shows an example method of assembling the rigid flex structure inaccordance with aspects of the present disclosure.

FIG. 10 shows an example cost comparison between a standard approachversus approach in accordance with aspects of the present disclosure.

DETAILED DESCRIPTION

In the description that follows, like components have been given thesame reference numerals, regardless of whether they are shown indifferent embodiments. To illustrate an embodiment(s) of the presentdisclosure in a clear and concise manner, the drawings may notnecessarily be to scale and certain features may be shown in somewhatschematic form. Features that are described and/or illustrated withrespect to one embodiment may be used in the same way or in a similarway in one or more other embodiments and/or in combination with orinstead of the features of the other embodiments.

The conventional rigid flex assembly procedure can be greatly simplifiedby employing reactive acrylic adhesive coupled with low pressure toolingplates to provide the bonding and process geometries, The lower pressure(2-5 psi gauge pressure) coupled with the lower cure temperature (23°C.) will constrain the adhesive from lowering its viscosity asdemonstrated in the standard high temperature high pressure method.Furthermore, as the viscosity of the adhesive decreases (the hightemperature and high pressure approach), the melting glue mix acts as alubricant requiring either additional process steps to minimizemovement, or tooling to confine the movement of the lubricated adhesive.Aspects of this approach (room temperature/low contact pressure method)for fabricating a rigid flex assembly will require about 30 minutes oftooling material set-up time and about 30 minutes of bonding cure timeonce the assembly is fabricated within the low pressure tooling plates.The corresponding time for the standard method is a total of threehours.

In general, there are several reasons why the low pressure, roomtemperature lamination process in accordance with aspects of the presentdisclosure are superior to the conventional approach of using hightemperatures and pressures. First, reducing internal part stressimproves low temperature performance and enables longer flex cable life.Room temperature cure of the adhesive will establish lower inner layerstress between the plastic layers and the adhesive. This stress occurswhen the adhesive melts and forms to the plastic. At higher temperatures(conventional method) the least stress is exhibited when operated athigh temperature. At room temperature, the conventional method seesstresses in the plastic and adhesive laminate. This is caused by the CTE(coefficient of thermal expansion) mismatch and material dimensionalchange from processing temperature to room temperature or below.

Second, dimensional stability can be improved during fabrication. Alladhesives when heated lower their viscosity for a short time periodprior to polymer linkages before establishing the full cure adhesivestate. During this lower transition viscosity stage the adhesive acts asa lubricant. As the adhesive wets against the plastic layers, thelubrication effect of the adhesive will allow relative movement betweenthe various layers of the part. In the present approach, roomtemperature is the only thermal energy imparted. Therefore, the loweringadhesive viscosity is kept to a minimum, the lubricating factor isnegated and the relative registration between the layers is maintained.

Third, cycle time/labor content can be reduced. Conventional rigid flexmethods require the platens and press to be heated along with thefabricated part. When the laminated flex cable is removed, substantialtime must be allowed for the assembly and manufacturing aids to coolprior to platen disassembly and part dc-flashing. The present approachis done at room temperature which requires no heating process or cureand eliminates the post laminate cool down period. De-flashing of thepart can be done at the end of the lamination cure cycle.

Further, equipment requirements and costs can he reduced. Roomtemperature processing eliminates the need for heating ovens. Moreover,reduced cycle time, labor content and less equipment requirementssignificantly reduces cost.

FIG. 1 shows an exploded view of an example flex portion of a rigid flexstructure in accordance with aspects of the present disclosure. The flexportion of the rigid flex structure, shown generally as 100, comprisestop coverlay 105 having top side 105A and bottom side 105B, intermediatelayer 110 having top side 110A and bottom side 110B, intermediate layer115 having top side 115A and bottom side 115B and bottom coverlay 120having top side 120A and bottom side 120B. A “coverlay” is a nonadhesive dielectric that contains no electronic conductors. Its purposeis to mechanically and chemically protect features and circuits that arebonded underneath. FIG. 1 shows two intermediate layers for illustrationonly. More than one intermediate layer may be used in the assembly ofthe instant rigid flex structure. For example, layers 105, 110, 115and/or 120 can include a polymer, such as polyimide, which is a polymerof imide monomers. Examples of polyimide films include APICAL® PolyimideFilm sold by the Kaneka Corporation, KAPTON® sold by DuPont, UPILEX®sold by Ube Industries and KAPTREX sold by Professional Plastics. Othersuitable polyimide films may also be used. Layers 110 and 115 caninclude conductive trace regions 125 and one or more attachment lands130. Layers 105 and 120 can include one or more registration holes 135.Coverlays 105 and 120 are arranged to provide environmental protectionand dielectric insulation to the innerlayer.

In some aspects, intermediate layers 110 and 115 can includeelectrically-conductive surface such as copper, gold, nickel, silver,aluminum, composites of copper and tin or lead alloy, composites ofcopper, nickel and antimony, composites of copper, nickel and gold, andcomposites of nickel, gold or silver. The electrically-conductivesurface can be arranged on a top side, a bottom side or both of thepolyimide film. This intermediate layers can then be imaged and etchedto form the conductive pattern using conventional printed wiring boardprocedures and equipment. The electrical conductor layers can bepatterned as desired by photo printing the desired circuit pattern onthe material normally utilizing a negative photo resist. After the photoprinting operation, the unwanted copper is etched and the electricalconductors are established in the desired pattern on the coppersubstrate.

In some aspects, first layer of thermoplastic film can be arranged onthe top of the rigid flex circuit board and a second layer ofthermoplastic film can be arranged on the bottom of the rigid circuitboard. In some aspects, the thermoplastic film can include afluoropolymer resin.

FIG. 2 shows the view of FIG. 1 with the addition of an adhesive appliedto the flex portion of the rigid flex structure. The flex portion of therigid flex structure, shown generally as 200, include the addition ofadhesive 205 that can be applied to top side 110A of intermediate layer110, top side 115A of intermediate layer 115 and top side 120A of bottomcoverlay 120. Alternatively or additionally, adhesive 205 may be appliedto bottom side 105B of top coverlay 105, bottom side 110B ofintermediate layer 110, bottom side 115B of intermediate layer 115.Adhesive 205 can include an acrylic-based adhesive, a room temperaturecuring silicone adhesive, a solvent blocked silicone adhesive, aflexible anaerobic adhesive, or a frozen preform adhesive. In aspectswhere a frozen preform adhesive is used, a frozen “B” stage preform canbe used by selecting any of the acrylic, silicone or anaerobic adhesivewhich requires an “A” component and “B” component mixture which is thenformed prior to use pressed into a sheet and stored at about −40 degreesC. For example: mixture of 5 parts “A” to 1 part “B” Arathane 5753adhesive or 1 part “A” to 1 part “B” of the 3M DP 810 adhesive. Press toa flat sheet and store at −40 degrees C. prior to use.

FIG. 3 shows an example low pressure and room temperature bondingapparatus for use with the flex portion of the rigid flex structure ofFIG. 2. Rubber mat and release film 305 can be positioned on either sideof flex portion of the rigid flex structure 200, which can be placedbetween top platen 315 and bottom platen 320. Top platen 315 can be ofsufficient size and mass to impart a pressure of about 1 to 10 psi, forexample about 5 psi, to structure 200 while at about room temperature,for example between about 20° and 25° C. Weight 325 can be added to topplaten 315 if top platen 315 is not of sufficient size and mass toimpart the required pressure on structure 200. Pressure can be appliedto structure 200 for about 30 minutes until structure 200 issufficiently formed.

FIG. 4 shows a result of the bonding process of FIG. 3. Layers 105, 110,115 and 120 are bonded together and drill/metallizing via/tooling holes135 are formed by standard rigid flex fabrication methods.

FIG. 5 shows an example exploded view of a portion of a rigid flexstructure including both the rigid and the flexible sections, showngenerally as 500. Intermediate flex layer 505 having top side 505A andbottom side 505B can be bordered on one side by rigid layer 510, havingtop side 510A and bottom side 510B, at top side 505A and by rigid layer515 having top side 515A and bottom side 515B at bottom side 505B.Intermediate flex layer 505 can be bordered on another side by rigidlayer 520 having top side 520A and bottom side 520B at top side 505A andby rigid layer 525 having top side 525A and bottom side 525B at bottomside 505B. Likewise, intermediate flex layer 530 having top side 530Aand bottom side 530B can be bordered on one side at top side 530A bybottom side 515B of rigid layer 515 and by rigid layer 535 having topside 535A and bottom side 535B at bottom side 530B. Intermediate flexlayer 530 can be bordered on another side at top side 530A by bottomside 525B of rigid layer 525 and by rigid layer 540 having top side 540Aand bottom side 540B at bottom side 530B. Top cover layer 545 having topside 545A and bottom side 545B can be positioned over top side 510A, topside 520A and top side 505A. Bottom cover layer 550 having top side 550Aand bottom side 550B can be positioned under bottom side 535B, bottomside 540B and bottom side 530B. One or more registration holes 560 canbe arranged on the various layers of structure 500. FIG. 5 shows twointermediate layers for illustration only. More than one intermediatelayer may be used in the assembly of the present rigid flex structure.

FIG. 6 shows the view of FIG. 5 with the addition of an adhesive appliedto portions of structure 500. Adhesive 605 can be applied in a mannersimilar to that described in relation to FIG. 2 such that adhesive 605can be applied to top side 505A of intermediate layer 505, top side 530Aof intermediate layer 530, top side 515A of rigid layer 515, top side525A of rigid layer 525, top side 535A of rigid layer 535, top side 540Aof rigid layer 540 and top side 550A of cover layer 550. Alternativelyor additionally, adhesive 605 may be applied to bottom side 545B of topcoverlay 545, bottom side 510B of rigid layer 510, bottom side 520B ofrigid layer 520, bottom side 515B of rigid layer 515, bottom side 525Bof rigid layer 525, bottom side 505B of flex layer 505, bottom side 530Bof flex layer 530, bottom side 535B of rigid layer 535 and bottom side540B of rigid layer 540. Adhesive 605 can include an acrylic-basedadhesive, a room temperature curing silicone adhesive, a solvent blockedsilicone adhesive, a flexible anaerobic adhesive, or a frozen preformadhesive.

FIG. 7 shows an example low pressure and room temperature bondingapparatus for use with the structure 500 of FIG. 6. Top rubber mat 705and bottom rubber mat 710 can be positioned on either side of structure500, which can be placed between top platen 715 and bottom platen 720.Top platen 715 can be of sufficient size and mass to impart a pressureof about 1 to 10 psi, for example about 5 psi, to structure 200 while atabout room temperature, for example between about 20° and 25° C. Weight725 can be added to top platen 715 if top platen 715 is not ofsufficient size and mass to impart the required pressure on structure500. Pressure can be applied to structure 200 for about 30 minutes untilstructure 500 is sufficiently formed.

FIG. 8 shows a result of the bonding process of FIG. 7. Flex section 705is bordered by rigid section 710 and 715 and are bonded together. 720 isone or more of the conductive path formed by the drilled/metalized viaholes.

FIG. 9 shows an example method of assembling the rigid flex structure inaccordance with aspects of the present disclosure. At 905, the innerlayers of the Kapton copper two sided laminate are imaged according to apredetermined circuit board configuration. At 910, the top and bottomsurface of the cover lays are prepared and imaged. At 915, a smallamount of adhesive, such as DP 810 sold by 3M, is applied in the middleof the bottom coverlay. The amount may be 5-25 grams. DP 810 is a roomtemperature, fast curing, flexible acrylic adhesive, which tends to bebeneficial for the present low pressure, room temperature laminationprocess. At 920, the next layer of inner layers is positioned upon theadhesive and treated again with another layer of adhesive. At 925, theprocess is repeated until all inner layers are stacked and treated andthe top coverlay is positioned upon the stack. At 930, a layer ofrelease film, such as FEP (fluorinated ethylene propylene resin) releasefilm and rubber mat, are positioned on the bottom of the stack. At 935,a layer of FEP release film and rubber are positioned on the top of thestack. At 940, two rigid platens are positioned on top and underneaththe FEP Flex stack. At 945, the platens are positioned in a clamp atbetween 2 to 5 psi. At 950, the assembly is allowed to room temperaturecure for 30 minutes. At 955, flex assembly is removed from clampedplatens and FEP. At 960, excessive flash or adhesive are trimmed fromflex assembly.

Cost Analysis Standard High Temperature High Contact Pressure MethodAssumptions

-   -   Assembly labor cost=$75/hour    -   Cost of new adhesive=$50/flex assembly    -   Fabrication of inner layers=4 labor hours    -   Lamination time=8-16 hours    -   High pressure tooling plates manufacturing aide=3 hours        (fabrication time)    -   Inspection time after lamination:=½ hour    -   Flash removal=15 minutes/flex assembly    -   Level of expected flex assembly repair about 10%    -   Touch-up and Repair=Difficult—typically exceeds the cost of        fabricating another flex assembly.

Approach in accordance with aspects of the present disclosure—RoomTemperature Low Contact Pressure Flex cable Assembly Method Assumptions

-   -   Assembly labor cost=$75/hour    -   Cost of new adhesive=$10/flex assembly    -   Fabrication in inner layers=4 labor hours    -   Lamination time=30 minutes    -   Low pressure clamping manufacturing aide=1 hour (fabrication        time)    -   Inspection time after lamination=½ hour    -   Flash removal=15 minutes/flex assembly    -   Level of expected flex assembly repair about 10%    -   Touch-up and Repair=Difficult—typically exceeds the cost of        fabricating another flex assembly.

As shown in FIG. 10, the implementation of the manufacturing process inaccordance with aspects of the present disclosure can reduce the cost ofmanufacturing a rigid flex cable assembly and reduce cycle time. Theunit cost savings is easily calculated at $1,146 or a total savings of$1.15M for 1,000 units built. Cycle time reduction is 1.7 days. Over thecourse of several years and multiple programs, 10,000 units may bemanufactured with a corresponding total savings of $11.5M.

In general, various products can be formed by the present low pressure,room temperature process. These products can include rigid flexcircuits, flexible cable interconnects, energy absorptive base plates,packaging/container for shipping applications and various automotiveproducts.

Although the above disclosure discusses what is currently considered tobe a variety of useful embodiments, it is to be understood that suchdetail is solely for that purpose, and that the appended claims are notlimited to the disclosed embodiments, but, on the contrary, are intendedto cover modifications and equivalent arrangements that are within thespirit and scope of the appended claims.

What is being claimed is:
 1. A product formed by a method comprising: arranging one or more layers of a polyimide film having an electrically-conductive surface into a stack; applying an adhesive to the one or more layers of polyimide film; arranging a top and a bottom polyimide film to a top portion and a bottom portion of the stack to form a rigid flex circuit board; disposing the rigid flex circuit board between a top platen and a bottom platen, wherein a flexible portion of the rigid flex circuit board is exposed; and selecting a suitable pressure and temperature at which to form the rigid flex assembly such that the one or more layers of polyimide film do not move with respect to one another during the fabrication, wherein the product is selected from the group consisting of rigid flex circuits, flexible cable interconnects, energy absorptive base plates, packaging/container for shipping applications, automotive products and combinations thereof.
 2. The product of claim 1, wherein the pressure is between about 1 psi and 10 psi at a temperature between about 20° and 25° C.
 3. The product of claim 1, wherein the electrically-conductive surface is selected from the group consisting of: copper, gold, nickel, silver, aluminum, composites of copper and tin or lead alloy, composites of copper, nickel and antimony, composites of copper, nickel and gold, and composites of nickel, gold and silver.
 4. The product of claim 1, wherein the electrically-conductive surface is arranged on a top side, a bottom side or both of the polyimide film.
 5. The product of claim 1, wherein the pressure is between about 2 psi and 5 psi.
 6. The product of claim 1, wherein the pressure is applied for about 30 minutes.
 7. The product of claim 1, wherein the adhesive is selected from the group consisting of an acrylic-based adhesive, a room temperature curing silicone adhesive, a solvent blocked silicone adhesive, a flexible anaerobic adhesive and a frozen preform adhesive.
 8. The product of claim I, wherein the method further comprises arranging a first layer of thermoplastic film to the top of rigid flex circuit board and a second layer of thermoplastic film to the bottom of the rigid circuit board.
 9. The product of claim 8, wherein e thermoplastic film includes a fluoropolymer resin.
 10. The product of claim 1, wherein the method further comprises preparing the electrically-conductive surface according to a predetermined circuit configuration.
 11. A product formed by a method comprising: forming a rigid flex cable assembly including a stack of one or more layers of a polyimide film having an electrically-conductive surface and an adhesive between the one or more layers of polyimide film; disposing the rigid flex cable assembly between a top platen and a bottom platen, wherein a flexible portion of the rigid flex cable assembly is exposed; and applying a suitable pressure at a suitable temperature to the rigid flex cable assembly such that the one or more layers of polyimide film do not move with respect to one another during the fabrication, wherein the product is selected from the group consisting of rigid flex circuits, flexible cable interconnects, energy absorptive base plates, packaging/container for shipping applications, automotive products and combinations thereof.
 12. The product of claim 11, wherein the pressure is between 1 psi and 10 psi.
 13. The product of claim 11, wherein the temperature is about 23° C.
 14. The product of claim 11, wherein the electrically-conductive surface is selected from the group consisting of copper, gold, nickel, silver, aluminum, composites of copper and tin or lead alloy, composites of copper, nickel and antimony, composites of copper, nickel and gold, and composites of nickel, gold and silver.
 15. The product of claim 11, wherein the electrically-conductive surface is arranged on a top side, a bottom side, or both, of the polyimide film.
 16. The product of claim 11, wherein the pressure is applied for about 30 minutes.
 17. The product of claim 11, wherein the adhesive is selected from the group consisting of an acrylic-based adhesive, a room temperature curing silicone adhesive, a solvent blocked silicone adhesive, a flexible anaerobic adhesive and a frozen preform adhesive. 